The (meth)acrylic acid and (meth)acrylic esters are easily-polymerizable compounds. When the (meth)acrylic acid or (meth)acrylic esters are stored in a storage tank for receiving or discharging the (meth)acrylic acid and (meth)acrylic esters for a long period of time, undesirable polymerization thereof tends to be frequently caused in the storage tank, resulting in problems such as production of insoluble polymers.
For the purpose of preventing production of the insoluble polymers in the storage tank, a polymerization inhibitor is added to the (meth)acrylic acid or (meth)acrylic esters stored therein. However, the polymerization inhibitor added is present only in a liquid phase in the storage tank, but is absent in a gas phase therein. In order to prevent production of polymers in such a gas phase, there are conventionally known the methods of blowing or sucking, as a polymerization-inhibiting gas, an oxygen-containing gas, for example, a mixed gas of an inert gas such as nitrogen, carbon dioxide gas and argon with air, or air solely into the gas phase portion of a storage facility (Japanese Patent Application Laid-open No. 2003-231661).
However, in fact, the above conventional methods have failed to completely prevent occurrence of troubles based on polymers of the (meth)acrylic acid or (meth)acrylic esters. According to the present inventors' knowledge, one reason therefor is that air flowing into the storage tank upon discharging the (meth)acrylic acid or (meth)acrylic esters therefrom contains suspended particulate matters (SPM). The suspended particulate matters induce polymerization of the (meth)acrylic acid or (meth)acrylic esters, resulting in production of insoluble polymers thereof.